Oooohhh this is a good one to think about...
For the first part... I think Mia would be one of the first tbh. I mean, she always complains about the serving size in Japan, so she'll constantly get bigger and bigger plates, so she's on the fast track for health issues I feel.

As for who would have it the worst... It's my usual suspect Yoshiko lol. I think other than her atrocious luck guaranteeing an early heart attack... I believe she would be extra delusional about her issues and actively refuse any medical assistance I feel, making it worse for her

Like her pounding/erratic heartbeat is a sign of her fallen angel powers growing, her gastrointestinal issues she passes off as her mortal form struggling with all the mana she's receiving, her sleep apnea she believes to be the work of God's angels trying to get her, etc.

Ooohhh this is kinda a tough one....

While the most obvious trait would be his Courage (King of Braves and all that), one that I love is his dedication: while it's mostly shown as a dedication to protect people, it all started as his dedication towards his dream: to be an astronaut and reach Jupiter and beyond, which was very nice to see in a hot-blooded protag that wasn't always just "I was kinda aimless before the events of the story"

Another big one is his brotherly bond with Mamoru.... Idk it's nice to see him be an older brother figure to the young kid, which let's his goofier moments shine + in GGG v B, when they get all drunk and sappy together talking about their lovers (makes him forget he can sober up instantly as an Evoluder lol)

Also I'm just a sucker for a long haired kinda bishonen dude, but that's another thing entirely lol

Sorry this one took so long, I got so into this ask that it got long and work was over lol

Any for the LL's:
Yoshiko absolutely adores her growing sinful body, so she absolutely will be extra horny and gluttonous over her gains, and will be loud and proud about it, no shame what so ever in her behavior, to almost everyone's embarrassment as she lets her gut hang free, let's out belches in public

GnY Yohane however, I feel will do the opposite and do all in her power to reject the idea that she is fat, just... simply a little bloated or something despite struggling to get anywhere and constantly needs to be taken care of by her partner

Hanamaru I feel wouldn't exactly care for her weight gain, it's just the price to pay for tasty food and she is willing to pay that price lol, BUT I feel if it's with a partner (YHMR lol) then she would be way more into weight gain past 350 lbs

I love the thought of Dia just being a pure unapologetic fatass/super horny over her growing body, but I also adore the thought of a nervous Dia as well, like yes you can eat all that you want, we won't judge you if your tights rip, or buttons pop off

Other than some occasional horny thoughts (fat demanding cat Ai....) Honestly I feel most of my Niji favs would just be the same, with a hint of gluttony to them... Though Kasumin would be really proud of her flab and wants to show it off every stretch mark and the cellulite on her body (to Shizuku's embarrassment)

Non LL's:
This one makes me crazy, but I like the thought of Guy being a little (6ft tall) fat house cat and just laying around, wanting to be spoiled after the events of GGG v B, but I also enjoy the thought of him purposely gaining insane amounts of weight cuz it helps his defense (punches to the gut by a cybernetic soldier don't hurt when you're an Evoluder + have a massive gut to take the blow), so he gets really defensive about his body, but he tries not to think about why it feels so good when he rubs his gut, or when he feels how tight his body is against his armor

Lapis Fire Emblem would be horny about her body every second that she isn't working or thinking about Rosado

Akiha Tohno would hate being a fatass at first, but then after a while (when Shiki says he doesn't hate it) will fully lean into it, and be very prideful over her growing body, saying it's a symbol of her power or to show the status/power of the Tohno family

Akane Shinjo would hate her flab/complain about it I feel, but make no effort to actually lose it
Like not even "tried to exercise a couple of times but gave up", just didn't even try

A throwback to a game that I haven't thought about in a long while:
I like the thought of Makoto Naegi (yes him) being a fatass, but also being super mushy about it, like "yeah being fat means we're at peace/on the right track of rebuilding society" or for Naegiri "I'm just full of my wife's love💕"

Anyway, sorry for the long ask, but hope this answers your question lol

Thank you for the ask, am glad to hear it!
I will explain one other concrete detail that I feel is important:
It's a college AU, but mostly because it's a good excuse for Dia to be alone and at a low point: moving away to go to college, separated from Aqours, and this pressure to excel in school in an unfamiliar place + from her family (except Ruby) to still come back and lead the estate leads her to kinda spiral into being a mess + gains, so she distances herself from everyone as a result

To answer the Riko question: at first she feels super conflicted on Dia's gain for multiple reasons, one of which I absolutely believe she's into feeder/feeder doujins, but since they haven't properly talked about their relationship, she feels like that she is taking advantage of Dia, while Dia feels the opposite and feels like she is taking advantage of Riko's kind and caring nature, so things get awkward for a while 😊

As for with Ruby.... Definitely Ruby is one of the first to try to reach out to Dia during her low point, but sadly isn't able to help her sis like Riko could, so she would beat herself up over not being there for her sis again

I do plan for the sisters to properly reconcile, but for a while it's also a little strained cuz Dia feels awful for "abandoning" Ruby, but is still gathering the courage to properly talk to her about everything, while Ruby completely understands why Dia would run, she can't help but feel a little frustrated at being thrusted into this role, + worries that she would end up blowing up at her sis when they meet in person again

Sorry for the long dump, but once again thank you for the ask!

hi ho!! plans so far have been working out well for me, thanks for asking!!
For new songs, been listening to Tomori's new single MAYBLUES and the new YYY single, but NGL have been in a strong Niji mood recently so Eternal Light has been played alot lol

And as for YohaMaru......
hard for me to express exactly why (instinct I guess lol), but one aspect I always love is how they give each other the courage to be themselves/pursue their dreams
More obviously shown in their GnY Duo song, their time as kids were very important memories to them, and while Yohane may not have succeeded in her dream, it still gave Hanamaru the courage to pursue her own, and now a fun little reverse of Hanamaru (+ everyone she introduced to Yohane) helping Yohane pursue her dream

thank you for the ask!